The present invention concerns a brake pressure control device for a road vehicle with hydraulic multi-circuit brake system, in particular a twin-circuit brake system with static brake circuits, into which brake system pressures proportional to the pedal force can be connected by pedal actuation of a brake unit, with an anti-lock brake system designed for dynamically stable deceleration behavior of the vehicle and with a sensor device recording the position of the brake pedal or of a brake unit element connected to the motion of the brake pedal, which sensor device generates electrical output signals characteristic of the instantaneous positions of the brake pedal.
It is possible to generate from the processing of the aforementioned output signals, via an electronic control unit in accordance with the present invention, control signals for an electrically controllable brake pressure setting device. The control of the setting device makes it possible to connect into the front wheel brakes, at least, a higher brake pressure than the expected brake pressure value associated with the instantaneous pedal position. Control of the brake pressure setting device in the sense of the increased brake pressure deployment is initiated at least whenever the speed .phi. with which the pedal position changes during a brake pressure build-up actuation is greater than a specified threshold value .phi..sub.s. At least one buffer reservoir controllable, via a valve, by output signals of the electronic control unit is provided in accordance with the present invention and can be pressurized against a lower return force than that corresponding to a reaction force resulting from a reaction of the brake pressure on the brake unit.
A brake pressure control device is shown in DE 34 44 827 A1. A tandem main cylinder with hydraulic brake booster is the brake unit. The brake booster of the brake unit is a differential cylinder whose piston rod acts axially on the primary piston of the main cylinder, to whose primary outlet pressure space is connected to one brake circuit of the brake system. The primary outlet pressure space has an axially movable boundary formed by the primary piston and a secondary piston which also forms the axial boundary, movable on one side, of the secondary outlet pressure space of the tandem main cylinder. The second brake circuit of the brake system is connected to the secondary outlet pressure space of the tandem main cylinder.
A drive pressure space on the pedal side and a back pressure space on the main cylinder side have a pressure-tight boundary relative to one another formed by the booster piston. A brake valve which can be actuated by the brake pedal is integrated in the booster piston. The actuation of this brake valve permits a pressure proportional to the pedal force to be connected into the drive pressure space of the brake booster, which produces the amplification effect when, simultaneously, the back pressure space is relieved, via a relief valve, to the storage reservoir of the auxiliary pressure source from whose outlet pressure the booster drive pressure is derived by way of the brake valve.
A pressure modulator is also provided as the brake pressure control element configured as a static pressure converter which has an outlet pressure space permanently connected to the main brake pipe of the brake circuit supplied with brake pressure via the primary outlet pressure space and emerging from the primary outlet pressure space of the tandem main cylinder. In the outlet pressure space, a pressure increased relative to the control pressure can be applied to the connected brake circuit by pressurization of a control pressure space of the pressure converter. It is possible to connect, with valve control, the outlet pressure of the brake valve also present in the drive pressure space of the brake booster into this control pressure space. The piston of the pressure modulator is pressed by a return spring into its basic position associated with maximum volume of the outlet pressure space of the pressure modulator and is held in this position if and as long as the valve-controlled control pressure space is relieved of pressure.
The modulator piston is also pressed into its basic position by the pressure present in the primary outlet pressure space of the tandem main cylinder and therefore also present in the outlet pressure space of the pressure modulator. This pressure is monitored by a pressure transducer. The position of the booster piston, and thus also the position of the primary piston of the tandem main cylinder, is monitored by a displacement signal generator. A pedal position signal generator can be provided as an alternative to the piston position signal generator.
In the known brake pressure control device, the pedal displacement/brake pressure characteristic of the brake unit can be changed by the valve-controlled pressurization of the drive pressure space of the pressure converter. This does not, however, apply to the pedal force/brake pressure characteristic, and therefore also not to the boosting of the braking force because the reaction on the brake pedal resulting from a certain brake pressure is always the same regardless of whether the pressure modulator is or is not pressurized. The conventional brake pressure control device is therefore not suitable for applications or braking situations in which high braking forces are desirable even at low actuation forces, for example in a situation which demands full braking with high vehicle deceleration commencing as soon as possible, but nevertheless controllable.
Similar problems occur in another known brake pressure control device described in DE 35 11 535 A1 which also operates with a degree of boosting specified by the design of a hydraulic brake booster but which, in contrast to the brake pressure control device of DE 34 44 827 A1, is a brake system with dynamic brake circuits whose tandem main cylinder in normal braking operations, i.e. with intact brake booster, is only used for position selection. Brake fluid displaced from the main cylinder is accepted by a buffer reservoir provided for pedal displacement simulation, and the tandem main cylinder is used only for brake pressure generation in the case of a failed brake booster, in which case the brake circuits act as static brake circuits.
An object of the present invention is to provide a brake unit for a mode of operation which offers the possibility of changing the brake pressure/actuating force relationship as a function of how rapidly the driver actuates the brake pedal, in particular of increasing it when the driver actuates the brake system very rapidly.
This object has been achieved according to the present invention by a valve-controlled buffer reservoir be connected by an output signal of the electronic control unit activating the brake pressure setting device, to that pressure outlet of the brake unit which is associated with the brake circuit in which the increased level brake pressure deployment occurs and the brake unit outlet pressure space connected to the buffer reservoir is, under valve control, shut off from the brake circuit subjected to the increased brake pressure deployment, on actuation of the brake pressure control device.
It is thus possible to operate the brake system with different values of the brake pressure/actuation force relationship such that, for example initiated by a signal which is generated by the pedal actuation speed having exceeded a threshold value, a high brake pressure is already connected to the wheel brakes in the initial phase of a braking action, in the course of which only a small fraction of the actuation stroke of the brake pedal has been utilized. The further evolution of the high brake pressure is then controlled in relation to the further actuation of the brake pedal. As an example, it is maintained if the driver continues to actuate the brake pedal and is only reduced when the driver has let the brake pedal come back by a minimum amount. Even if, in such an automatic braking action, the maximum possible brake pressure is connected to the wheel brakes from the outset, there remains possible a pedal displacement which can be monitored and which is necessary for recognizing the driver's wishes. This is because brake fluid can continue to be displaced from the brake unit into the buffer reservoir in accordance with the present invention and the pedal feel appearing after the onset of automatic brake pressure control is substantially the same as in the case of a normal braking action so that the driver is not irritated by the automatically controlled transition to automatic brake pressure control.
It is desirable for the buffer reservoir acting, so to speak, as the displacement simulator to be provided for that brake circuit which accepts the larger brake fluid volume, i.e. for the front axle brake circuit in the case of a brake system with front axle/rear axle brake circuit division. The result of this is, of course, that the pedal displacement which can be used for the pedal "feel" is somewhat reduced. If, however, a buffer reservoir is provided for each of the two brake circuits, the complete pedal displacement can be also used for control of the brake pressure control device.
In a currently preferred embodiment of the brake pressure setting device, one or more buffer reservoirs are designed as return spring piston reservoirs. Such reservoirs can, in turn, be equipped with displacement sensors to record their piston displacements. These can be used to check the pedal displacement sensor and, in turn, can be used for sensing the driver's wishes. It is also possible to equip such reservoirs with preloading devices in order to set a spring preload particularly suitable for the driver and thereby t adjust the brake system to the stature of the driver.
If the particular buffer reservoir is provided with a preloading device which, while the brake pressure setting device is not actuated, keeps the preload of the reservoir spring to a value which corresponds to the force which results from the reservoir piston being subjected to the outlet pressure of the connected outlet pressure space of the brake unit, then, for example, a brake pedal is prevented from collapsing due to pressure accepted by the buffer reservoir in the case where the brake pressure control device is actuated while brake pressure has already been built up in the wheel brakes by a braking action initiated with low pedal force. Furthermore, a reduction in such a pedal reaction can be at least moderated simply by a preload provided from the outset by the buffer reservoir springs.
The collapse can be slowed, hence avoiding a frightening pedal reaction with pulsed shut-off of the buffer reservoir, by using pulse-controlled solenoid valves, from the particular outlet pressure space of the brake unit.
A preloading device is formed locating the reservoir spring between two pistons, one of which forms an axially movable boundary of the reservoir chamber while the second piston forms an axially movable boundary of a control pressure space into which a pressure corresponding to the pressure in the connected outlet pressure space of the brake unit is always connected. The outlet pressure of the brake unit appears fundamentally suitable for this purpose but could be problematic for safety reasons.
For such safety reasons, it may be more advantageous for the pressure follow-up in to the control pressure space of the buffer reservoir to take controlled by a pulse-controlled follow-up valve for connection of either the outlet pressure of the auxiliary pressure source or this control pressure space to the unpressurized storage reservoir of the auxiliary pressure source. In principle, such control is possible by 3/2 solenoid valves which hav a basic position in which the control pressure space is connected to the storage reservoir of the auxiliar pressure source and is shut off against its high pressure outlet and which have an excited position in which the outlet pressure space is connected to the high pressure outlet of the auxiliary pressure source but is shut off against the storage reservoir thereof.
It is more advantageous, however, for such a follow-up valve to be a 3/3-way solenoid valve which ca be driven by a first control signal (with a relatively low control current strength of, for example, 3 A) from its basic position 0, in which the control pressure space is connected to the storage reservoir of the auxiliary pressure source and is shut off from the latter's high pressure outlet, into a first excited position I (i.e. a shut-off position) in which the control pressure space is shut off from both the high pressur outlet of the auxiliary pressure source and its storage reservoir, and which can be driven by a control signal (with relatively higher control current strength of, for example, 6 A) into a second excited position II, in which the control pressure space of the buffer reservoir is connected to the high pressure outlet of the auxiliary pressure source and is shut off from its storage reservoir.
In one currently preferred configuration of the brake pressure control device for a brake system with front axle/rear axle brake circuit division, at least one pressure sensor generates an electrical output signal characteristic of the pressure in the outlet pressure space, of the brake unit, associated with the front axle brake circuit I, and at least one pressure sensor generates an electrical output signal characteristic of the pressure in at least one of the front wheel brakes. By using the output signals of these sensors, it is then possible in a simple way to make a comparison between the brake unit outlet pressure, as the required value for the front axle brake circuit, and the pressure prevailing in this circuit in order to achieve an appropriate control of the pressure connection into the control pressure space of the pressure modulator with follow-up spring preload.
The same is possible in the case of brake systems with any given brake circuit division if, for each outlet pressure space of the brake unit, a pressure sensor is provided which generates an output signal characteristic of the particular outlet pressure and at least one pressure sensor is associated with each of the brake circuits I and II.
A particularly simple type of control for the pressure connection into the control pressure spaces of the pressure modulators is then also possible if a pressure sensor recording the pressure in the control pressure space of the particular pressure modulator and a pressure sensor recording the pressure in the connected outlet pressure space of the brake unit are provided.
In a preferred configuration of the brake pressur control device, at least one pressure modulator is provided as the brake pressure setting element of the brake pressure control device. In this pressure modulator, a piston forms a movable pressure-tight boundary between a control pressure space and an outlet pressure space. When the control pressure space is subjected to the valve-controlled outlet pressure of the auxiliary pressure source, the piston can be displaced in the direction of a brake pressure build-up in the brake circuit connected to the outlet pressure space. By way of valve-controlled connection of the control pressure space to the storage reservoir of the auxiliary pressure source, the piston can be displaced in the direction of a pressure reduction in the connected brake circuit. Furthermore, the piston is driven by a preloaded, spring-elastic return element into its basic position corresponding to minimum volume of the control pressure space or maximum volume of the outlet pressure space.
In this case, it is again particularly advantageous for control pressure deployment if brake pressure control valves provided for connecting the control pressure space of the particular pressure modulator to the high pressure outlet of the auxiliary pressure source or, alternatively, to its storage reservoir, are 3/3-way solenoid valves, whose basic position 0 is a flow position shutting off the control pressure space of the particular pressure modulator from the high pressure outlet of the auxiliary pressure source but connecting it to the storage reservoir. Their firs excited position I, taken up on excitation of the control magnets by a control signal of relatively low control current strength of, for example, 3 A, is a shut-off position in which the control pressure space is shut off from both the high pressure outlet of the auxiliary pressure source and its storage reservoir. The second excited position II of the 3/3-way solenoid valves, taken up on excitation of their control magnets by a control signal of relatively higher excitation current strength of, for example, 6 A, is again a flow position in which the control pressure space of the pressure modulator is connected to the high pressure outlet of the auxiliary pressure source but is shut off from its storage reservoir.
Such valves are also suitable as function control valves of the brake pressure control device if their basic position 0 is a flow position shutting off the particular outlet pressure space of the brake unit from the reservoir chamber of the buffer reservoir but connecting it to the continuing section of the main brake pipe of the particular brake circuit. These valves, on excitation of their control magnets by a control current of relatively low control current strength of, for example, 3 A, take up a first excited position I (i.e. a shut-off position) in which the outlet pressure space of the brake unit is shut off from both the reservoir chamber of the particular buffer reservoir and the continuing main brake pipe section and on excitation of their control magnet by a relatively higher control current strength of, for example, 6 A, take up a second excited position II, which is again a flow position, in which the outlet pressure space of the brake unit is connected to the reservoir chamber of the particular buffer reservoir but is shut off from the continuing main brake pipe section.
Because the function control valves remain held in the position provided for this purpose during their activation period (a position in which the particular outlet pressure space of the brake unit is connected to the associated buffer reservoir) once the brake pressure control device has responded, these valves can also be two-position valves, i.e. as 3/2-way solenoid valves which only have alternative flow positions and can therefore be switched from one flow position to the other by a single control signal of defined control current strength. This permits simplification of the position selection.
Such a 3/2-way solenoid valve with simple position selection can be replaced by an even simpler 2/2-way solenoid valve and an hydraulically controlled, additional 2/2-way valve if the particular pressure modulator whose outlet pressure space is connected to the brake pipe section continuing to the anti-lock brake system or to the wheel brakes of the particular brake circuit I or II is connected to the pressure outlet of the brake unit via a valve. The basic position of this valve is a flow position connecting the pressure outlet of the brake unit (in the basic position of the modulator piston) to the outlet pressure space of the pressure modulator. Controlled by the motions of the modulator piston, the valve reaches its shut-off position after a small fraction of the pressure build-up stroke of the modulator piston, and is brought back to its open position by a return of the modulator piston to its basic position. A pressure modulator configured in such a way is analogous with the construction and function of a single-circuit main cylinder in which the pressure-controlled or displacement-controlled 2/2-way valve can be brought about either by providing a follow-up hole which can be shut off or connected by the piston motions from or to the pressure outlet of the brake unit or by providing a central valve integrated in the piston of the pressure modulator can be provided as is already known from conventional main cylinders.
The above described embodiments the brake pressure control device in accordance with the present invention can be constructed as an additional unit to an otherwise conventional motor vehicle brake system. If required, the vehicle can be equipped with this unit as a retrofit item. In the case, however, of vehicles which are to be equipped from the outset with an anti-lock brake system, an acceleration skid regulation device (ASR) and possibly also with a device for the electronic control of a more or less ideal front axle/rear axle brake fcrce distribution, it is also desirable to integrate, again from the outset, the brake pressure control device according to the present invention into such a complex feed-back and open-loop control system, i.e. by using the components already available for the anti-lock brake system and the acceleration skid regulation device. This is possible in a simple manner in that a device analogous to the acceleration skid regulation device for the driven vehicle wheels is provided in order to subject the brake circuit of the non-driven vehicle wheels to brake pressure. The brake pressure control can be achieved by control of the analogous device as a function of the brake pedal position and its changes.
In a vehicle with front axle/rear axle brake circuit division, rear wheel drive and an acceleration skid regulation device (in which the return pump of the rear axle brake circuit is used as the auxiliary pressure source, a booster pump being provided to supply it with brake fluid), the brake pressure control device can be simply implemented by also connecting a booster pump upstream of the return pump of the front axle brake circuit and by providing, in addition to the hydraulic unit of this feed-back and open-loop control system, a subsidiary hydraulic unit. This subsidiary hydraulic unit includes the two buffer reservoirs and the function control valves of the brake pressure control subsidiary unit, which are 2/2-way solenoid valves provided for connecting the buffer reservoirs to and shutting them off from the outlet pressure spaces of the brake unit.